Intense, if flawed, hardcore SF flick.
Check out this review from Mark R. Kelly, which breaks down all the film's science flaws and implausibilities. And try this review by Gary Westfhal, which does a nice job of placing Sunshine within the context and history of "space suit" films.
This could have been a truly great science fiction film if it wasn't for the silly plot twist during the last act. Still, extremely worth it for the visuals.
Check out video interviews with director below. I think he's a great director, but maybe understands film making more than he fully understands science fiction--or only understand science fiction through the movies, with no broader view of the genre.